Re: [PATCH v2 3/5] mm: Default implementation of arch_wants_pte_order()

[Yin Fengwei]

On 7/5/23 09:23, Yu Zhao wrote:
> On Tue, Jul 4, 2023 at 6:36 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> On 04/07/2023 04:59, Yu Zhao wrote:
>>> On Mon, Jul 3, 2023 at 9:02 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>>>>
>>>> On Mon, Jul 3, 2023 at 8:23 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 7/3/2023 9:53 PM, Ryan Roberts wrote:
>>>>>> arch_wants_pte_order() can be overridden by the arch to return the
>>>>>> preferred folio order for pte-mapped memory. This is useful as some
>>>>>> architectures (e.g. arm64) can coalesce TLB entries when the physical
>>>>>> memory is suitably contiguous.
>>>>>>
>>>>>> The first user for this hint will be FLEXIBLE_THP, which aims to
>>>>>> allocate large folios for anonymous memory to reduce page faults and
>>>>>> other per-page operation costs.
>>>>>>
>>>>>> Here we add the default implementation of the function, used when the
>>>>>> architecture does not define it, which returns the order corresponding
>>>>>> to 64K.
>>>>>>
>>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>>>> ---
>>>>>>  include/linux/pgtable.h | 13 +++++++++++++
>>>>>>  1 file changed, 13 insertions(+)
>>>>>>
>>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>>> index a661a17173fa..f7e38598f20b 100644
>>>>>> --- a/include/linux/pgtable.h
>>>>>> +++ b/include/linux/pgtable.h
>>>>>> @@ -13,6 +13,7 @@
>>>>>>  #include <linux/errno.h>
>>>>>>  #include <asm-generic/pgtable_uffd.h>
>>>>>>  #include <linux/page_table_check.h>
>>>>>> +#include <linux/sizes.h>
>>>>>>
>>>>>>  #if 5 - defined(__PAGETABLE_P4D_FOLDED) - defined(__PAGETABLE_PUD_FOLDED) - \
>>>>>>       defined(__PAGETABLE_PMD_FOLDED) != CONFIG_PGTABLE_LEVELS
>>>>>> @@ -336,6 +337,18 @@ static inline bool arch_has_hw_pte_young(void)
>>>>>>  }
>>>>>>  #endif
>>>>>>
>>>>>> +#ifndef arch_wants_pte_order
>>>>>> +/*
>>>>>> + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
>>>>>> + * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
>>>>>> + * to be at least order-2.
>>>>>> + */
>>>>>> +static inline int arch_wants_pte_order(struct vm_area_struct *vma)
>>>>>> +{
>>>>>> +     return ilog2(SZ_64K >> PAGE_SHIFT);
>>>>> Default value which is not related with any silicon may be: PAGE_ALLOC_COSTLY_ORDER?
>>>>>
>>>>> Also, current pcp list support cache page with order 0...PAGE_ALLOC_COSTLY_ORDER, 9.
>>>>> If the pcp could cover the page, the pressure to zone lock will be reduced by pcp.
>>>>
>>>> The value of PAGE_ALLOC_COSTLY_ORDER is reasonable but again it's a
>>>> s/w policy not a h/w preference. Besides, I don't think we can include
>>>> mmzone.h in pgtable.h.
>>>
>>> I think we can make a compromise:
>>> 1. change the default implementation of arch_has_hw_pte_young() to return 0, and
>>> 2. in memory.c, we can try PAGE_ALLOC_COSTLY_ORDER for archs that
>>> don't override arch_has_hw_pte_young(), or if its return value is too
>>> large to fit.
>>> This should also take care of the regression, right?
>>
>> I think you are suggesting that we use 0 as a sentinel which we then translate
>> to PAGE_ALLOC_COSTLY_ORDER? I already have a max_anon_folio_order() function in
>> memory.c (actually it is currently a macro defined as arch_wants_pte_order()).
>>
>> So it would become (I'll talk about the vma concern separately in the thread
>> where you raised it):
>>
>> static inline int max_anon_folio_order(struct vm_area_struct *vma)
>> {
>>         int order = arch_wants_pte_order(vma);
>>
>>         return order ? order : PAGE_ALLOC_COSTLY_ORDER;
>> }
>>
>> Correct?
>>
>> I don't see how it fixes the regression (assume you're talking about
>> Speedometer) though? On arm64 arch_wants_pte_order() will still be returning
>> order-4.
> 
> Here is what I was actually suggesting -- I think the problem was
> because contpte is a bit too large for that benchmark and for the page
> allocator too, unfortunately. The following allows one retry (32KB)
> before fallback to order 0 when using contpte (64KB). There is no
> retry for HPA (16KB) and other archs.
> 
> +       int preferred = arch_wants_pte_order(vma) ? : PAGE_ALLOC_COSTLY_ORDER;
> +       int orders[] = {
> +               preferred,
> +               preferred > PAGE_ALLOC_COSTLY_ORDER ?
> PAGE_ALLOC_COSTLY_ORDER : 0,
> +               0,
> +       };
> 
> I'm attaching a patch which fills in the two helpers I left empty here [1].
> 
> Would the above work for Intel, Fengwei?
PAGE_ALLOC_COSTLY_ORDER is Intel preferred because it fits the most common
Intel system. So yes. This works for Intel.


Regards
Yin, Fengwei

> 
> (AMD wouldn't need to override arch_wants_pte_order() since PTE
> coalescing on Zen is also PAGE_ALLOC_COSTLY_ORDER.)
> 
> [1] https://lore.kernel.org/linux-mm/CAOUHufaK82K8Sa35T7z3=gkm4GB0cWD3aqeZF6mYx82v7cOTeA@xxxxxxxxxxxxxx/2-anon_folios.patch